Image processing has many applications. One such application is in a digital copier. A digital copier typically has different modes for copying text or an image. The different modes, text or letter mode vs. photo mode, include special image processing techniques such as sharpening or smoothing of the scanned document, thinning of text, despeckeling, etc. Problems arise if the copied document has text and image content. In order to apply different techniques to text and image regions of a document, a segmentation of the document or a classification of the pixel of the scanned document is needed. Without out this classification, an oversmoothing of text in the photo-mode may occur and an enhancement of halftone noise in the text mode may occur. A consequence of this enhancement of halftone noise in the text mode can be the creation of severe Moiré patterns after resolution reduction. Such artifacts are observed in many current digital copiers. A typical imaging path for digital gray scale copier is demonstrated in FIG. 1.
Referring to FIG. 1, a scanned image 100 is input into processing block 101, which changes gray values in the scanned image to compensate for scanner bias through lookup tables or other point operation (e.g., gamma correction). Next, different regions in the scanned image are classified (e.g., text, halftone, continuous tone, etc.) (processing block 102). The pixels are then filtered by processing block 103 depending on the classified pixel. Such filtering may include smoothing or sharpening of pixels based on their classification. After filtering, the pixels undergo up and/or downsampling by processing block 104. The pixels may also be subjected to resizing for enlarging/reducing. Next, the gray values of the pixels are changed through look up tables or other point operation (e.g., gamma correction) to compensate for printer bias (processing block 105). Once changing of the gray values has been done, halftoning is performed on the pixels (processing block 106). For non-copier applications, halftoning might be replaced with another output formatting step, the result of which is processed image 107.
Single components of the digital imaging copier path are prior art, such as contrast enhancement of the scanned image or halftoning for printing. See U.S. Pat. No. 5,883,973, entitled “Method and Apparatus for Processing a Document by Segmentation into Text and Image Areas,” issued Mar. 16, 1999 to Pasovici and Shu, and U.S. Pat. No. 5,805,721, entitled “Method and Apparatus for Contrast Enhancement,” issued Sep. 8, 1998 to Vuylsteke and Schoeters. There are also publications on image enhancement with wavelets that include denoising and contrast enhancement. For example, Zong, Laine, Geiser, and Wilson, “De-Noising and Contrast Enhancement via Wavelet Shrinkage and Non-linear Adaptive Gain,” Proceedings of the SPIE, Vol. 2762, pgs. 566-574, 1996. Image enhancement using the Laplacian pyramid to add high frequency content to the image is also known. For example, see U.S. Pat. No. 5,717,789, entitled “Image Enhancement by Non-linear Extrapolation in Frequency Space,” issued February 1998 to Anderson and Greenspan.